Dedicator of cytokinesis 8 (DOCK8) is a guanine nucleotide exchange factor (GEF) that coordinates the actin cytoskeleton response to mitogenic and chemokine signals, most notably by regulating the activity of the small GTP binding protein cell division cycle 42 (CDC42) [1]. Its deficiency due to loss-of-function deletions and mutations in DOCK8 underlies the majority of cases of the autosomal recessive form of the hyper IgE syndrome (AR-HIES), a combined primary immunodeficiency disease (PID) characterized by susceptibility to viral bacterial and fungal infections, severe atopy, early onset autoimmunity and malignancy [3-5]. The phenotype of DOCK8 deficiency overlaps with that of the autosomal dominant form of the hyper IgE syndrome (AD-HIES), due to heterozygous loss of function mutations in STAT3. Both diseases exhibit hyper IgE phenotype, and susceptibility to bacterial and candidal infections. AD-HIES is associated with failure of Th17 cell differentiation due to ineffective STAT3 activation. Our preliminary studies revealed a similar defect in Th17 differentiation in DOCK8-deficent subjects. Furthermore, we demonstrate that DOCK8 constitutively binds to STAT3 and promotes its phosphorylation at the regulatory tyrosine (Y)705, as well as phospho-(p)STAT3 nuclear translocation and transcriptional function. DOCK8 deficiency impaired pSTAT3 activation and translocation, indicating a critical role for DOCK8 in STAT3 function. Furthermore, a point mutation that abrogated DOCK8 GEF activity and which recapitulated key attributes of DOCK8 deficiency also impaired STAT3 activation, indicative of the latter's dependence on CDC42 activity. Accordingly we hypothesize that DOCK8 acts as a critical regulator for STAT3 function by controlling its phosphotyrosine phosphorylation and cytoplasmic to nuclear shuttling, a novel and unexpected mechanism by which DOCK8 controls Th17 differentiation and other attributes of STAT3 activities in immune cells. We propose to examine mechanisms of STAT3-DOCK8 interaction and the role of CDC42 in this process. We also propose to examine mechanisms by which DOCK8 shuttles between the cytosol and the nucleus, including its binding to ?- and ?-importins and to exportin1. We will examine the capacity of DOCK8 to associate with gene targets in STAT3-dependent and independent manner to modulate gene expression. Finally, we will test the hypothesis that DOCK8 deficiency skews the T follicular helper (TFH) cell response towards a Th2 cell-like phenotype and disrupts the differentiation of TFR cells and their regulation of TFH cells by both STAT3-dependent and independent mechanisms, leading to the promotion of IgE and Th2 responses. These studies will elucidate fundamental mechanisms by which DOCK8 may regulate the immune response, the interruption of which precipitates distinct clinical phenotypes associated with DOCK8 deficiency.